1. Technical Field
This invention relates to methods and apparatus for non-invasively determining biological tissue oxygenation utilizing near-infrared spectroscopy (NIRS) techniques in general, and to sensors for use with such techniques in particular.
2. Background Information
Near-infrared spectroscopy is an optical spectrophotometric method that can be used to continuously monitor tissue oxygenation. The NIRS method is based on the principle that light in the near-infrared range (700 nm to 1,000 nm) can pass easily through skin, bone and other tissues where it encounters hemoglobin located mainly within micro-circulation passages; e.g., capillaries, arterioles, and venuoles. Hemoglobin exposed to light in the near-infrared range has specific absorption spectra that varies depending on its oxidation state; i.e., oxyhemoglobin (HbO2) and deoxyhemoglobin (Hb) each act as a distinct chromophore. By using light sources that transmit near-infrared light at specific different wavelengths, and measuring changes in transmitted or reflected light attenuation, concentration changes of the oxyhemoglobin (HbO2) and deoxyhemoglobin (Hb) can be monitored. The ability to continually monitor cerebral oxygenation levels, for example, is particularly valuable for those patients subject to a condition in which oxygenation levels in the brain may be compromised, leading to brain damage or death.
NIRS type sensors typically include at least one light source and one or more light detectors for detecting reflected or transmitted light. The light signal is created and sensed in cooperation with a NIRS system that includes a processor and an algorithm for processing signals and the data contained therein. U.S. Pat. No. 7,047,054, which is commonly assigned with the present application to CAS Medical Systems, Inc. of Branford, Conn., discloses an example of such a sensor. Light sources such as light emitting diodes (LEDs) or laser diodes that produce light emissions in the wavelength range of 700-1000 nm are typically used. A photodiode or other light detector is used to detect light reflected from or passed through the tissue being examined. The NIRS System cooperates with the light source(s) and the light detectors to create, detect and analyze the signals in terms of their intensity and wave properties. U.S. Pat. No. 6,456,862, and U.S. Pat. No. 7,072,701, both of which are commonly assigned to CAS Medical Systems, Inc., of Branford, Conn., disclose a methodology for analyzing such signals. U.S. Pat. Nos. 6,456,862, 7,047,054, and 7,072,701 are hereby incorporated by reference in their entirety.
The light emanating from the light source may be described as traveling along a “mean optical path” through the tissue under examination. The “mean optical path” represents an idealized path traveled by a predominant number of photons emanating from the light source and sensed by the detector, recognizing however that not all photons emanating from the light source will travel the mean optical path. The length of the mean optical path and the depth from the surface reached by the path are a function of the separation distance between the light source and the light detector and the geometry of the path. Several sources of research in NIRS technology provide that the mean optical path follows a “banana-shaped” path.
Meaningful cerebral oxygenation information is collected from light interrogating brain tissue (e.g., passing through, reflecting from, absorbed by, etc.). To non-invasively access the brain tissue, however, the light signal must pass through extracerebral tissue (e.g., scalp, skull, etc.) before and after interrogating the brain tissue. A light signal traveling within any biological medium (e.g., tissue, fluid, etc.) will attenuate, and the amount of attenuation is a function of the medium. In the case of a mean optical path that non-invasively accesses brain tissue, the attenuation attributable to the extracerebral tissue does not yield useful information with respect to the cerebral oxygenation. Consequently, it is desirable to account for the signal attenuation attributable to extracerebral tissue, so that the attenuation attributable to the brain tissue can be distinguished and analyzed.
It is known to use a NIRS sensor, having a pair of light detectors specifically spaced apart from a light source as a means to account for extracerebral tissue; i.e., a first separation distance between the light source and a “near” light detector, and a second separation distance between the light source and a “far” light detector. U.S. Pat. No. 5,482,034, for example, discloses a method for spectrophotometric cerebral oximetry that purports to collect optical response data that represents purely intrinsic brain tissue; i.e., without the effects that result from the interrogating light spectra passing through the structure and substances disposed outwardly of the brain.
According to the '034 patent, light of selected wavelengths is introduced into the subject at a “source” location, and is sensed at first and second light detection locations that are spaced from one another and spaced from the source location by unequal, but preferably comparable and not greatly disproportionate, first and second distances, thereby defining unequal first and second mean optical paths extending between the source and the first and second light detector locations. The mean optical path extending between the source and the near light-detector location is selected so that the internal region encompassed by that mean optical path includes not only the full thickness of the overlying tissue, etc. disposed between the outer surface and the interior region to be examined, but also at least a small portion of the physiological substance disposed within the internal region. The signals sensed at the near detector and at the far detector are then processed to obtain optical response data which particularly characterizes only the tissue of the internal region disposed between the two mean optical paths.
A disadvantage of a method such as that disclosed in the '034 patent is that the two mean optical path lengths of comparable and not greatly disproportionate length limit the amount of tissue interrogation, and therefore the available information.
The '034 patent also identifies and distinguishes the subject matter claimed therein from U.S. Pat. No. 5,217,013, which was issued earlier and assigned to the same assignee as the '034 patent: Somanetics Corporation, of Troy, Mich. According to the '034 patent, the '013 patent discloses a sensor arrangement wherein a near detector is positioned very near a light source, and further discloses an “optimum” light source—near detector separation distance of about eight (8) millimeters, and a light source—far detector separation of about twenty-three (23) millimeters.
The '034 patent “reassesses” and teaches away from the source-receiver positioning disclosed in the '013 patent, however, indicating that the “near” detector should be located at least about twenty to thirty (20-30) millimeters from the source, and the “far” detector should be positioned at least about five to ten (5-10) millimeters distant from the “near” detector; i.e., closer to the “near” detector than to the source. Finally, the '034 patent discloses that locating the “far” detector more than forty (40) millimeters away from the source is not useful as a practical matter, with commercially available and economically feasible components.
The light source—near detector separation distance is not only important relative to the area of interrogation defined by the two optical mean paths, but is also significant relative to signal contamination from the light source itself. An optical shunt field surrounding a light source provides a light path through tissue (usually laterally through the skin surface) without absorption by chromophores such as hemoglobin in blood. The intensity of shunted light within the optical shunt field decreases with increasing distance from the light source. The optical shunt field extends out a distance from the light source, which distance is defined as the point at or beyond which the contamination from the optically shunted light is no longer of significance. If a light detector is placed within the optical shunt field surrounding the light source, the light detected from the shunt field will contain little or no spectrophotometric information of value from biological tissue, and can lead to erroneous calculation of biological tissue oxygenation.
What is needed, therefore, is an improved sensor and method for non-invasively determining the level of oxygen saturation within biological tissue, which sensor is configured to permit interrogation of, and the capture of signal from, a substantial amount of tissue, while at the same time accounting for attenuation attributable to extracerebral tissue, and also a sensor that limits or eliminates undesirable optical shunt effects surrounding the sensor's light source.